1. Field of the Invention
The present invention relates to a stabilizer for power system which is applied to an exciter of a synchronous generator.
2. Description of the Prior Art
FIG. 1 is a system configuration diagram showing the construction of a typical stabilizer for power system. Referring to the same Figure, reference numeral 1 designates a synchronous generator (hereinafter referred to as generator), numeral 2 designates a circuit breaker for tripping the generator 1 from an external power system 4, numeral 3 designates a main transformer disposed between the circuit breaker 2 and the external power system 4, numeral 5 designates a turbine shaft connected to the rotor of the generator 1, numeral 6 designates a transformer for detecting current from the generator 1, numeral 7 designates a transformer for detecting the terminal voltage of the generator 1, numeral 13 designates a converter for calculating output from the generator according to voltage and current detected by means of the transformers 6, 7, numeral 51 designates a power system stabilizer for generating auxiliary signals for automatic voltage regulator (AVR) by inputting the rotation speed of the rotor and the power generated in the generator 1, numeral 10 designates automatic voltage regulator (AVR) for stabilizing the terminal voltage in the generator 1, numeral 8 designates a transformer for supplying part of electric power generated in the generator 1 to a thyrister exciter 9a which excites the field winding of the generator 1.
When the terminal voltage of the generator 1 detected by the transformer 7 deviates from the reference value, the AVR 10 controls the thyrister exciter 9a so as to make the deviation from the reference value zero. The power system stabilizer 51 receives the output of the generator through the converter 13 and the rotation speed of the rotor. To improve the stability of the power system, the power system stabilizer 51 produces auxiliary signals from the output of the generator and the rotation speed of the rotor and supplies auxiliary signals to the AVR. The AVR 10 controls the thyrister exciter 9a according to the voltage deviation reflected by the auxiliary signals Generally, the power system stabilizer 51 uses one of the deviation of the generator output, the deviation of the rotation speed of the generator and the deviation of system frequency. The deviation mentioned here refers to difference to each reference value.
FIG. 2 is a block diagram showing a section containing the power system stabilizer and the excitation system (including the AVR and the exciter) in the conventional stabilizer for power system disclosed in, for example, Japanese Patent Publication No. 4-35975. The components shown in FIG. 2 constitute a two-parallel-type power system stabilizer (two-parallel-type PSS). Referring to the same Figure, numeral 31 designates a low-pass filter which receives the deviation signal of the rotation speed of the rotor of the generator 1 from an input terminal 23 and passes through only components below a predetermined frequency of the input signal. Numeral 22 designates a first power system stabilizer for producing an auxiliary signal relating to the deviation of the rotation speed according to the output of the low-pass filter 31. In the first power system stabilizer 22, numeral 22a designates a filter circuit for determining the reacting range of the input signal and usually has the transfer characteristic in the form of [Tr1.S/(1+Tr1.S)].[1/(1+Th1.S)]. Numeral 22b designates an amplifying/phase correcting circuit for compensating time lag of a regulator 26, an exciter 9, the generator 1 and the like, and usually has the transfer characteristic in the form of Kp.(1+TP2.S)/(1+TP1.S). Numeral 22c designates a limiter circuit for limiting the output of the first power system stabilizer 22 so that it is on appropriate signal level from the viewpoint of the overall excitation system.
Reference numeral 32 designates a high-pass filter receiving the output deviation signal from the input terminal 29 and passing through only components above a predetermined frequency of the input signal. Numeral 28 designates a second power system stabilizer for producing an auxiliary signal relating to the output deviation according to the output of the high-pass filter 32. In the second power system stabilizer 28, numeral 28a designates a filter circuit for determining the reacting range of the input signal and has the transfer characteristic in the form of [Tr2.S/(1+Tr2.S)].[1/(1+Th2.S)]. Numeral 28b designates an amplifying/phase correcting circuit and has the transfer characteristic in the form of Kw.(1+Tw2S)/(1+Tw1S). Numeral 28c designates a limiter circuit for limiting the output of the second power system stabilizer 28 so that it is on appropriate signal level from the viewpoint of the overall excitation system. Numeral 30 designates a subtractor circuit for subtracting the output of the second power system stabilizer 28 from the output of the first power system stabilizer 22.
Reference numeral 21 designates an input terminal to which a deviation from the reference value of the terminal voltage of the generator 1 is input. Numeral 25 designates an operation circuit for adding the voltage deviation from the input terminal 1 to the output of the subtractor circuit 30 and simultaneously subtracts the output of the dumping circuit 24 from the sum. Numeral 26 designates a regulator for controlling the exciter 9 according to the output of the operation circuit 25. Numeral 9 designates an exciter for exciting the field winding of the generator 1. Numeral 24 designates a dumping circuit for achieving feed-back of the output of the exciter 9 toward the input side of the regulator 26 in order to stabilize voltage control.
The filters 31, 32, the first power system stabilizer 22, the second power system stabilizer 28 and the subtractor circuit 30 correspond to the power system stabilizer 51. The operation circuit 25, the regulator 26 and the dumping circuit 24 correspond to the AVR. The exciter 9 corresponds to the thyrister exciter 9a.
Next, the operation of the stabilizer for power system will be described below. The power system stabilizer 51 generally uses one of the deviation of the generator output, the deviation of the rotation speed of the generator and the deviation of the system frequency. Herein the case in which the deviation of the rotation speed of the generator is input into the power system stabilizer 22 will be explained. When a rotation speed deviation signal is input to the input terminal 23, the low-pass filter 31 cuts off frequency components of over 5-10 rad/sec in order to remove influences of noise and twisting vibration from the signal. Further, the rotation speed deviation signal inputs to the amplifying/phase correcting circuit 22b after the dc component and high frequency component are eliminated by the filter circuit 22a. The amplifying/phase correcting circuit 22b amplifies the signal and corrects the phase thereof appropriately. The amplifying/phase correcting circuit 22b is set so as to perform phase lead correction. Then, the limiter circuit 22c limits the output of the amplifying/phase correcting circuit 22b so as to be below appropriate signal level from the viewpoint of the overall excitation system.
When a deviation occurs in the rotation speed of the generator, the output of the generator is also changed. Thus, an output deviation signal is input to the input terminal 29. The high-pass filter 32 eliminates frequency component of below 1-2 rad/sec in order to secure sufficient effect against frequency variation in such a range in which the first power system stabilizer 22 does not act effectively. The output deviation signal inputs to the amplifying/phase correcting circuit 28b after dc component and high frequency component are eliminated by the filter circuit 28a. The amplifying/phase correcting circuit 28b is set so as to perform phase lag correction. The limiter circuit 28c limits the output of the amplifying/phase correcting circuit 28b so as to be below appropriate signal level from the viewpoint of the overall excitation system.
The subtractor circuit 30 subtracts the output of the second power system stabilizer 28 from the output of the first power system stabilizer 22 and then outputs the result to the operation circuit 25. The operation circuit 25 adds the output of the subtractor circuit 30 to the deviation of the terminal voltage of the generator 1, the deviation being input through the input terminal 21. Then, the operation circuit 25 subtracts the output of the dumping circuit 24 and supplies the result to the regulator 26.
The aforementioned action of the two-parallel-type power system stabilizer take measures effectively to system unstableness which includes 1) vibration in low frequency system having a cycle of 3-5 sec, and 2) vibration in the generator having a cycle of about 1 sec. That is, the first power system stabilizer 22 to which a rotation speed deviation is input, having a characteristic for phase lead generates an auxiliary signal against low frequency vibration between power systems. The second power system stabilizer 28 to which a generator output deviation is input, having a characteristic for phase lag generates an auxiliary signal against output vibration between generators, having a cycle of about 1 sec. Then, these auxiliary signals are added to the excitation system so as to raise dumping performance in a plurality of electric power unstable modes.
The conventional stabilizer for power system is constructed in the aforementioned configuration. Thus, it functions effectively to a plurality of power unstable modes which can be preliminarily expected. However, if the number of the electric power unstable modes to be taken measures to is increased, it is necessary to increase the number of the power system stabilizers correspondingly.
Constants for use in the amplifying/phase correcting circuits 22b, 28b are design values corresponding to the relation between the output P of the generator 1 and the internal phase angle 6, the relation being linearized in the vicinity of a certain stable point. Thus, the power system stabilizer 51 operates effectively against little vibration in the vicinity of the design point (e.g., .delta.1 in FIG. 3). However, the conventional apparatus has such a problem that the stability of the power system cannot be secured when non-linearity becomes more intense (e.g., when the stability point is moved to .delta.2 in FIG. 3) caused by movement of the stability point is moved due to a change of the power system construction or a large scale load trip.